Continuous Evaporator

ABSTRACT

A continuous evaporator for a horizontally constructed waste heat steam generator is provided. The continuous evaporator includes a first evaporator heating surface having a plurality of essentially vertically arranged first steam generator tubes through which a flow medium can flow from bottom to top, and a second evaporator heating surface which is mounted downstream of the first evaporator heating surface on the flow medium side. The second evaporator heating surface includes a plurality of additional essentially vertically arranged second steam generator tubes through which a flow medium can flow from bottom to top. The second steam generator tubes are designed in such a manner that the average mass flow density which can be controlled in the full load operation does not fall below a predetermined minimum mass flow density in the second steam generator tubes.

The invention relates to a method for designing a once-through evaporator and to a once-through evaporator for a horizontally constructed waste heat steam generator with a first evaporator heating surface which incorporates a number of first steam generation tubes, the arrangement of which is essentially vertical and through which the flow is from the bottom to the top, and another second evaporator heating surface, which on the flow substance side is connected downstream from the first evaporator heating surface, which incorporates a further number of second steam generation tubes the arrangement of which is essentially vertical and through which the flow is from the bottom to the top.

In the case of a combined cycle gas turbine plant, the heat contained in the expanded working substance or heating gas from the gas turbine is utilized for the generation of steam for the steam turbine. The heat transfer is effected in a waste heat steam generator connected downstream from the gas turbine, in which it is usual to arrange a number of heating surfaces for the purpose of preheating water, for steam generation and for superheating steam. The heating surfaces are connected into the water-steam circuit of the steam turbine. The water-steam circuit usually incorporates several, e.g. three, pressure stages, where each of the pressure stages can have an evaporator heating surface.

For the steam generator connected downstream on the heating gas side from the gas turbine as a waste heat steam generator, several alternative design concepts can be considered, namely a design as a once-through steam generator, or a design as a recirculatory steam generator. In the case of a once-through steam generator the heating up of steam generation tubes, which are provided as evaporation tubes, results in the flow substance being evaporated in a single pass through the steam generation tubes. In contrast to this, in the case of a natural or forced circulation steam generator, the water which is fed around the circulation is only partially evaporated during its passage through the evaporator tubes. After the steam which has been generated has been separated off, the water which has not yet been evaporated is then fed once more to the same evaporator tubes for further evaporation.

Unlike a natural or forced circulation steam generator, a once-through steam generator is not subject to any pressure limitations. A high live steam pressure favors a high thermal efficiency, and hence low CO₂ emissions from a fossil-fuel fired power station. In addition, a once-through steam generator has, by comparison with a recirculatory steam generator, a simple construction and can thus be manufactured at particularly low cost. The use of a steam generator, designed in accordance with the once-through principle, as the waste heat steam generator for a combined cycle gas turbine plant is therefore particularly favorable for the achievement of a high overall efficiency for the combined cycle gas turbine plant together with simple construction.

A once-through steam generator which is designed as a waste heat steam generator can basically be engineered in one of two alternative forms of construction, namely as a vertical construction or as a horizontal construction. A once-through steam generator with a horizontal construction is then designed so that the heating substance or heating gas, for example the exhaust gas from the gas turbine, flows through it in an approximately horizontal direction, whereas a once-through steam generator with a vertical construction is designed so that the heating substance flows through it in an approximately vertical direction.

Unlike a once-through steam generator with a vertical construction, a once-through steam generator with a horizontal construction can be manufactured with particularly simple facilities, and with particularly low manufacturing and assembly costs. In this case, an uneven distribution of the flow substance can arise across the steam generation tubes, in particular within each individual row of tubes in the steam generation tubes of the second evaporator heating surface, said tubes being connected downstream on the flow substance side, leading to temperature imbalances and, because of different thermal expansions, to mechanical stresses. For this reason expansion bends, for example, have hitherto been incorporated to compensate for these stresses, in order to avoid damage to the waste heat steam generator. However, this measure can be technically comparatively expensive in the case of a waste heat steam generator with a horizontal construction.

The object underlying the invention is thus to specify a method for designing a once-through evaporator together with a once-through evaporator, for a waste heat steam generator of the type identified above, which has a particularly long service life while permitting a particularly simple construction.

In respect of the method, this object is achieved in accordance with the invention in that a minimum mass flow density is prescribed and the second steam generation tubes are designed in such a way that the mean mass flow density which is established through the second steam generation tubes when operating at full load does not fall below the prescribed minimum mass flow density.

The invention then starts from the consideration that it would be possible to achieve a particularly simple construction for the waste heat steam generator or once-through evaporator, as applicable, by the elimination of the previously-usual expansion bends. In doing so however, the mechanical stresses caused by the temperature imbalances in the steam generation tubes which are connected in parallel with one another in each individual row must be reduced in some other way. These occur, in particular, in the second evaporator surface, to which is admitted a water-steam mixture. The temperature imbalances are here caused by the different proportions of water and steam at the flow side entry to the individual tubes in a row of tubes, and the resulting different through-flow through these tubes. A static stabilization of the flow, and at the same time a particularly simple construction for the waste heat steam generator, can be achieved by direct modification of the parameters of the steam generation tubes in the second evaporator heating surface. Here, a reduction in the temperature imbalances can be achieved by designing the second steam generation tubes in such a way that the mean mass flow density which is established through the second steam generation tubes when operating at full load does not fall below a prescribed minimum mass flow density.

It is advantageous in this case if the value of the prescribed minimum mass flow density is 180 kg/m² s. That is, a design of the steam generation tubes to achieve such a choice of mass flow density leads to a particularly good static stabilization of the flow in each individual row of tubes in the second evaporator heating surface, and hence to a particularly good equalization of the temperature in steam generation tubes which are connected in parallel in each individual row of tubes in the second evaporator heating surface.

It has been recognized that this different mass flow density in the tubes is caused by a frictional pressure loss in the steam generation tubes which is small by comparison with the geodetic pressure loss. That is, a flow which has a high proportion of steam in the flow substance flows through individual steam generation tubes comparatively fast with a low frictional pressure loss, whereas a flow with a high proportion of water is disadvantaged by its greater geodetic pressure loss, caused by its mass, and can tend towards stagnation. In order to even out the through-flows, the frictional pressure loss should therefore be increased. This can be achieved in that the internal diameter of the second steam generation tubes is advantageously chosen in such a way that the mean mass flow density which is established when operating at full load does not fall below the prescribed minimum mass flow density.

The objective is further achieved by a once-through evaporator designed in accordance with the method cited above.

A reduction in the internal diameter for ensuring a minimum mass flow should not, however, be taken arbitrarily far. On the basis of various operating parameters there can be a minimum desirable diameter. So, for example, the surface of the steam generation tubes must permit adequate heat input. In this context, the steam generation tubes often also have external ribbing, which in turn requires a certain minimum diameter. A minimum thickness is also required on grounds of rigidity and stability. Not least, if the internal diameter is too small, the geodetic pressure loss of the water fraction of the flow substance can be so low that a reversal of the desired effect sets in, and a flow with a large proportion of water reaches, too high velocities in the parallel steam generation tubes. For this reason, the internal diameter of the second steam generation tubes should advantageously not be less than a minimum diameter, determined by reference to prescribed operating parameters.

It is advantageous if the internal diameter of the second steam generation tubes is then between 20 mm and 40 mm. That is, a choice of internal diameter in this range determines the mass flow density in the second steam generation tubes to be such that the frictional pressure loss in the steam generation tubes lies within a range for which a through-flow with a high proportion of water and a through-flow with a high proportion of steam lead to exit temperatures with comparatively small temperature differences. Consequently, the temperature differences within each row of tubes in the second evaporator heating surface are minimized, whereby the other operating prerequisites are satisfied at the same time.

In an advantageous embodiment, a number of second steam generation tubes are connected one after another on the heating gas side as rows of tubes. This makes it possible to use for the evaporator heating surface a larger number of steam generation tubes connected in parallel, which means a better heat input from the enlarged surface. However, the steam generation tubes which are arranged one after another in the direction of flow of the heating gas are then differently heated. Particularly in the steam generation tubes on the heating gas entry side, the flow substance is comparatively strongly heated. However, a through-flow which is matched to the heating can also be achieved in these steam generation tubes, by the design described for the steam generation tubes such that the mass flow density at full load does not drop below a minimum value. By this means, a particularly long service life is achieved for the waste heat steam generator by a simple construction.

In an advantageous embodiment, the first evaporator heating surface is connected downstream from the second evaporator heating surface on the heating gas side. This offers the advantage that the second evaporator heating surface, which is connected downstream on the flow substance side and is thus designed to further heat up a flow substance which has already been evaporated, also lies in a comparatively more strongly heated region of the heating gas duct.

A once-through evaporator of this type can expediently be used in a waste heat steam generator, and the waste heat steam generator used in a combined cycle gas turbine plant. In this case it is advantageous to connect the steam generator downstream on the heating gas side from a gas turbine. With this connection, a supplementary heat source can expediently be arranged behind the gas turbine, to raise the heating gas temperature.

The advantages achieved by the invention consist, in particular, in the fact that designing the second steam generation tubes in such a way that the mean mass flow density established through the second steam generation tubes when operating at full load does not fall below a prescribed minimum mass flow density achieves a static stabilization of the flow, and thus a reduction in the temperature differences between steam generation tubes connected in parallel and in the mechanical stresses which result therefrom. This makes the service life of the waste heat steam generator particularly long. An appropriate design of steam generation tubes enables further expensive technical measures such as expansion bends to be foregone, and thus at the same time permits a particularly simple cost-saving construction for the waste heat steam generator or combined cycle gas turbine power station, as applicable.

An exemplary embodiment of the invention is explained in more detail by reference to a drawing. The FIGURE here shows a simplified representation of a longitudinal section through a steam generator with a horizontal construction.

The once-through steam generator 1 for the waste heat steam generator 2 shown in the FIG. is connected downstream from a gas turbine, not shown here in more detail, on its exhaust gas side. The waste heat steam generator 2 has a surrounding wall 3 which forms a heating gas duct 5 through which the exhaust gas from the gas turbine can flow in an approximately horizontal direction as heating gas, as indicated by the arrows 4. Arranged in the heating gas duct 5 is a number of evaporator heating surfaces 8, 10, designed according to a once-through principle. In the exemplary embodiment shown in the FIG., each of two evaporator heating surfaces 8, 10 is shown, but a larger number of evaporator heating surfaces could also be provided.

Each of the evaporator heating surfaces 8, 10 shown in the FIG. incorporates a number of rows of tubes, 11 and 12 respectively, each in the nature of a nest of tubes, arranged behind each other in the direction of the heating gas. Each row of tubes 11, 12 incorporates in turn a number of steam generation tubes, 13 and 14 respectively, in each case arranged beside each other in the direction of the heating gas, of which in each case only one can be seen for each row of tubes 11, 12. The first steam generation tubes 13 of the first evaporator heating surface 8, which are arranged approximately vertically and connected in parallel so that a flow substance W can flow through them, are here connected on their output sides to an outlet collector 15 which is common to them. The second steam generation tubes 14 of the second evaporator heating surface 10, which are also arranged approximately vertically and connected in parallel so that a flow substance W can flow through them, are also connected on their output sides to an outlet collector 16 which is common to them. Here, a comparatively expensive collection system could also be provided for both the evaporator heating surfaces 8, 10. For flow purposes, the steam generation tubes 14 of the second evaporator heating surface 10 are connected downstream from the steam generation tubes 13 of the first evaporator heating surface 8, via a downpipe 17.

The evaporation system formed by the evaporator heating surfaces 8, 10 can have admitted to it the flow substance W which, in a single pass through the evaporation system, is evaporated and after it emerges from the second evaporator heating surface 10 is fed away as steam D. The evaporation system formed by the evaporator heating surfaces 8, 10 is connected into a steam turbine's water-steam circuit, which is not shown in more detail. In addition to the evaporation system which incorporates the evaporator heating surfaces 8, 10, the water-steam circuit of the steam turbine has connected into it a number of other heating surfaces 20, indicated schematically in the FIG. The heating surfaces 20 could be, for example, superheaters, medium-pressure evaporators, low-pressure evaporators and/or preheaters.

The second steam generation tubes 14 are now designed in such a way that the mass flow density does not fall below a minimum prescribed for full load as 180 kg/m² s. Here, their internal diameter is between 20 mm and 40 mm so that, on the one hand, the required operating parameters such as rigidity, heat input etc. are satisfied and, on the other hand, temperature imbalances within a row of tubes in the second evaporator heating surface 10 are minimized. This reduces the mechanical stress loadings on the waste heat steam generator 2, guaranteeing a particularly long service life and at the same time a simple construction due to the elimination of the previously usual expansion bends. 

1.-10. (canceled)
 11. A method for designing a continuous evaporator for a horizontally constructed waste heat steam generator, comprising: providing a first evaporator heating surface which incorporates a plurality of first steam generation tubes, a first arrangement of which is essentially vertical and through which a flow is from the bottom to the top; providing a second evaporator heating surface which incorporates a plurality of second steam generation tubes a second arrangement of which is essentially vertical and through which the flow is from the bottom to the top; and connecting the second evaporator heating surface downstream from the first evaporator heating surface on a flow substance side, wherein a minimum mass flow density is prescribed, and wherein a mean mass flow density established in the plurality of second steam generation tubes when operating at full load does not fall below the prescribed minimum mass flow density.
 12. The method as claimed in claim 11, wherein the prescribed minimum mass flow density is 180 kg/m² s.
 13. The method as claimed in claim 11, wherein an internal diameter of the plurality of second steam generation tubes is chosen such that the mean mass flow density which is established in the plurality of second steam generation tubes when operating at full load does not fall below the prescribed minimum mass flow density.
 14. The method as claimed in claim 13, wherein the internal diameter of the plurality of second steam generation tubes is not less than a minimum diameter determined on the basis of prescribed operating parameters.
 15. The method as claimed in claim 13, wherein the internal diameter of the plurality of second steam generation tubes includes a value between 20 and 40 mm.
 16. The method as claimed in claim 11, wherein the plurality of second steam generation tubes are connected one after another on a heating gas side as rows of tubes.
 17. A continuous evaporator, comprising: a first evaporator heating surface incorporating a plurality of first steam generation tubes; and a second evaporator heating surface incorporating a plurality of second steam generation tubes, wherein an arrangement of the plurality of first steam generation tubes and the plurality of second generation tubes is essentially vertical and through which a flow is from the bottom to the top, wherein the first evaporator heating surface is connected downstream from the second evaporator heating surface on a heating gas side, wherein a minimum mass flow density is prescribed, and wherein a mean mass flow density established in the plurality of second steam generation tubes when operating at full load does not fall below the prescribed minimum mass flow density.
 18. The continuous evaporator as claimed in claim 17, wherein the prescribed minimum mass flow density is 180 kg/m² s.
 19. The continuous evaporator as claimed in claim 17, wherein an internal diameter of the plurality of second steam generation tubes is chosen such that the mean mass flow density which is established in the plurality of second steam generation tubes when operating at full load does not fall below the prescribed minimum mass flow density.
 20. The continuous evaporator as claimed in claim 19, wherein the internal diameter of the plurality of second steam generation tubes is not less than a minimum diameter determined on the basis of prescribed operating parameters.
 21. The continuous evaporator as claimed in claim 19, wherein the internal diameter of the plurality of second steam generation tubes includes a value between 20 and 40 mm.
 22. The continuous evaporator as claimed in claim 17, wherein the plurality of second steam generation tubes are connected one after another on the heating gas side as rows of tubes.
 23. A waste heat steam generator, comprising: a continuous evaporator as claimed in claim
 17. 24. The waste heat steam generator as claimed in claim 23, wherein upstream from the waste heat steam generator on the heating gas side is connected a gas turbine.
 25. The waste heat steam generator as claimed in claim 23, wherein an internal diameter of the plurality of second steam generation tubes is chosen such that the mean mass flow density which is established in the plurality of second steam generation tubes when operating at full load does not fall below the prescribed minimum mass flow density.
 26. The waste heat steam generator as claimed in claim 25, wherein the internal diameter of the plurality of second steam generation tubes is not less than a minimum diameter determined on the basis of prescribed operating parameters.
 27. The waste heat steam generator as claimed in claim 25, wherein the internal diameter of the plurality of second steam generation tubes includes a value between 20 and 40 mm.
 28. The waste heat steam generator as claimed in claim 23, wherein the plurality of second steam generation tubes are connected one after another on the heating gas side as rows of tubes. 